1. Field of the Invention
The present invention relates to the control or elimination of High Temperature Oxidation (HTO) of aluminum surfaces, especially those of alloys which contain magnesium, when such surfaces are exposed to heat treatment.
2. Description of the Prior Art
High temperature oxidation (HTO) has long been a problem in the aluminum industry which extends itself into a number of areas such as vacuum brazing and casting. In the bright anodic finishing of automobile bumpers, very mild HTO can cause very severe problems in the specularity and image quality of the surface.
It is theorized, that during solution heat-treatment, hydrated oxides on the metal surface break down, releasing nascent hydrogen. The hydrogen diffuses into the metal surface and coalesces into bubbles of hydrogen gas. When bright finished, the bubbles of gas are exposed producing an unacceptable appearance.
Methods which have been used to reduce or eliminate HTO include dry inoculation of furnace atmospheres with fluoroborate compounds and removal of the surface by mechanical or chemical means prior to solution heat-treatment. The former furnace innoculation methods are described in detail in U.S. Pat. Nos. 2,885,313; 2,885,315 and 2,885,316. Although such methods are very useful in controlling HTO they pose obvious environmental, materials handling and cost problems because of the requirement of filling the entire internal volume of a furnace with a vaporized fluorinated compound.
Japanese Patent Application No. 50014-1979 suggest the use of certain sulfate and fluoride compounds to inhibit blistering of aluminum materials prior to heat treatment.
Accordingly, research has continued for more advantageous methods of controlling or eliminating HTO.